1. Field of the Invention
The present invention relates to a flexible liquid crystal display device.
2. Description of the Related Art
In recent years, so-called flat panel displays have widely been used as display devices in various fields. As one of such display devices, a liquid crystal display device has broadly been used in the field of mobile displays such as cellular phones and PDAs by taking advantage of its advantages of thinness, lightness and low power consumption. In this field, there is a growing demand for thinner and lighter displays and flexibility for realizing wearable displays is also required. Therefore, use of a flexible substrate made of plastic or the like instead of glass has been under review.
As shown in a sectional view of FIG. 23, a liquid crystal display device 100 includes a first flexible substrate 101, a second flexible substrate 102 and a liquid crystal layer 104 sealed between the substrates 101 and 102 by a rectangular frame-like sealing member 103. The thickness of the liquid crystal layer 104 is controlled by spacers 105 provided between the substrates 101 and 102. The spacers 105 may be glass or resin particles of uniform particle size or columnar photo spacers. FIG. 23 shows a plurality of photo spacers 105 formed on the first flexible substrate 101.
An area enclosed with the sealing member 103 functions as a display area which contributes to display and a peripheral area outside the display area is a frame area which does not contribute to display. When the liquid crystal display device 100 is in a flat state, the width L1 of the display area on the first flexible substrate 101 is equal to the width L2 of the display area on the second flexible substrate 102 as shown in FIG. 23.
When the liquid crystal display device 100 is bent such that the first flexible substrate 101 protrudes outward, tensile force is applied to the outside first flexible substrate 101 and compression force is applied to the inside second flexible substrate 102 as shown in a sectional view of FIG. 24 due to the difference between the radii of curvature R1 and R2 of the first and second flexible substrates 101 and 102.
As a result, when the liquid crystal display device 100 is bent, the compression force is applied in the thickness direction to part of the liquid crystal layer in the middle of the display area. At the same time, part of the second flexible substrate 102 in the display area close to the sealing member 103 is convexly deformed (protrudes in the direction toward the center of curvature) and the cell gap increases. Further, peel stress is applied to part of the second flexible substrate 102 near the sealing member 103 in the direction away from the first flexible substrate 101.
Due to the existence of the spacers 105 such as photo spacers, the cell gap in the middle of the display area is not reduced below the thickness of the spacers 105. On the other hand, in the vicinity of the sealing member 103, the cell gap and the peel stress on the sealing member 103 become larger as the radii of curvature R1 and R2 become smaller. Since the stress is continuously applied to the sealing member 103, the liquid crystal display device 100 becomes less reliable. Further, the increase of the cell gap leads to reduction of display quality in the vicinity of the sealing member 103.
As a solution to the cell gap failure, a liquid crystal pool is provided between the display area and the sealing member (see, for example, Japanese Unexamined Patent Publication No. 2000-199891). To be more specific, as shown in a perspective view of FIG. 25, a liquid crystal display device 100 according to Japanese Unexamined Patent Publication No. 2000-199891 includes liquid crystal pools P1 formed in areas W1 and W2 on the edges of a bent display panel in the bending direction of the panel (hereinafter merely referred to as the edge areas). In each of the edge areas W1 and W2, linear sealing members 108 are arranged inside the sealing member 103 in a broken line pattern extending parallel to the sealing member 103. An area partitioned by the linear sealing members 108 and the sealing member 103 outside the linear sealing members 108 is the liquid crystal pool P1. A display area 110 is formed between the broken lines made of the linear sealing members 108.
When the liquid crystal display device 100 of Japanese Unexamined Patent Publication No. 2000-199891 is bent, liquid crystal in the display area 110 flows toward the edge areas W1 and W2 and overflows into the liquid crystal pools P1 through gaps 108a between the linear sealing members 108. With this configuration, the possibility of occurrence of the cell gap failure in the vicinity of the sealing member 103 is reduced and the cell gap in the display area 110 is kept uniform for the purpose of preventing the reduction of display quality.
In general, the liquid crystal display device is a flexible display panel using a flexible plate. Therefore, the peel stress is easily applied to the sealing member due to local deformation not only when the panel is intentionally bent but also in usual work on the liquid crystal display device.
As a countermeasure to this, the thickness of the sealing member is set larger than the cell gap in the display area (See, for example, Japanese Unexamined Patent Publication No. 2001-13508 and Japanese Unexamined Patent Publication No. 58-193518). According to Japanese Unexamined Patent Publication No. 2001-13508, as shown in a sectional view of FIG. 22, the diameter of spacer particles 106 contained in the sealing member 103 is set larger than the thickness of the spacers 105 in the display area 110 between the first and second flexible substrates 101 and 102 such that the thickness of the sealing member 103 becomes larger than the cell gap in the display area 110. Likewise, the sealing member according to Japanese Unexamined Patent Publication No. 58-193518 is also thickened. This configuration is intended to improve the uniformity in display of the liquid crystal display panel using the flexible plate.
For the liquid crystal display device of Japanese Unexamined Patent Publication No. 2000-199891, it is impossible to prevent the increase of the cell gap in the vicinity of the linear sealing members due to the difference in radius of curvature between the first and second flexible substrates as explained with reference to FIG. 24. That is, the display quality may deteriorate in part of the display area near the linear sealing members due to the cell gap failure.
As to the liquid crystal display devices of Japanese Unexamined Patent Publication No. 2001-13508 and Japanese Unexamined Patent Publication No. 58-193518, part of the sealing member 103 in which the spacer 106 exists has the same thickness as the diameter of the spacer 106, while part of the sealing member 103 near the display area 110 is gradually reduced in thickness toward the display area 110 as shown in FIG. 22. This is because the sealing member 103 is spread to an inside area where the cell gap is smaller than the thickness of the spacer 106 when the plates are bonded. Therefore, part of the sealing member 103 near the interface with the liquid crystal layer 104 becomes thinner than the middle part thereof.
Specifically, the cell gap at an interface between the sealing member 103 and the display area 110 is substantially the same as that in the display area 110. Therefore, the peel stress is applied to the sealing member 103, which leads to the reduction in reliability of the liquid crystal display device.